The present invention relates to a new and improved method of experimentally determining the transmissivity of a particular optical medium, such as a particular region of the atmosphere.
One of the major problems in the field testing of flare systems has been the effects of atmospheric attenuation of light. Methods that have been developed and utilized to measure the transmissivity of a medium are complex, costly, and difficult to set up and operate. This is particularly true in the case of a field test environment because light attenuation by the atmosphere causes extremely large errors in the determination of the source intensity.
In the past, the Army, Navy, and Air Force have relied on weather reports giving only a crude estimate of the limits of visibility to determine test conditions. Commercial transmissometers have been experimented with for assessing losses in source measurements in the field testing of flare systems and have been found to be impractical. In such cases, the complexity and cost of conducting the test or the accuracy and reliability of the data became a problem. Typically, tests simulating battlefield illumination conditions have operated neglecting atomospheric attenuation. This leads to errors in determining the intensity of the source as high as 50%.
This invention provides a method of measuring the atmospheric transmission of the actual test range (optical path) during the conduct of the test. Using this information, flare intensity measurements can then be corrected to true candlepower values. In addition, this technique utilizes equipment similar to that which has been provided for the conventional illumination measurements. Because of this, the cost and time involved with the transmission measurements are kept to a minimum. Illumination measurements in field flare testing are taken with photometers. They typically consist of a photocell-filter combination corrected to the visibility function (average response of the human eye) and associated electronics.
In accordance with the present invention: three radiation sensors, are positioned at three points P.sub.1, P.sub.2 and P.sub.3 aligned with each other and with a radiation source, such as an illuminating flare, and located at equal known distances d from each other, with the middle sensor at a distance D, greater than d, from the source; the amounts or levels of radiation E.sub.1, E.sub.2, and E.sub.3 simultaneously received by the three sensors from the source are recorded; and the transmissivity T, or transmission per mile, of the optical medium traversed by the radiation is calculated from the formula: ##EQU2## where the E values are in foot candles and d is in miles. Thus, the transmissivity can be determined without knowledge of either the source intensity or the distance D.